1. Field of the Invention
This invention relates to the general field of optical scanning to collect data and digital images from documents, and within that field to the more specific field of scanning wherein the colors printed on a sheet may be selectively processed so that key marks and response targets may be printed in the same color. Although the methods and apparatus described herein may be useful in other related tasks, the most common usage is likely to be in the processing and scoring of standardized assessment tests, and particularly for OMR (Optical Mark Recognition).
2. Description of the Prior Art
Traditional OMR processes, in which respondents place marks on forms or sheets, require that such forms be manufactured to precisely comply with very tight design tolerances. Most response sheets for use with traditional OMR systems have targets printed, typically in the shape of a circle, with a special ink to show respondents where to make marks. This special ink absorbs little or no light in the spectrum processed by the optical system, instead reflecting it to the scanning camera in close to the same intensity as a white area of the sheet. If no response mark is made in the target area, the scanning system produces a pixel image of the area as not having any mark, just as if the paper were blank throughout the area Many OMR systems use the red or infrared spectrum for processing and red ink for the targets, since the red ink highly reflects incident light in the infrared spectrum while pencil marks highly absorb the infrared light.
Sophisticated OMR systems also use key marks for location and orientation, including a special form of key marks known as timing tracks that indicate the scanning speed by the distance between track marks in the image. These key marks are printed in a second ink, typically black, that highly absorbs light in the appropriate color spectrum and therefore can be recognized and processed by the optical system. The ink used to print key marks may also be used to print other marks to identify the type of form and/or to identify the particular document. The various inks used for the targets, and other inks used for key marks and identification marks, are well known in the art. However, the requirement of using two separate inks on the same form and the degree of printer control necessary to ensure that the two inks are properly in register of each other makes OMR form printing an exacting and expensive process.
An advantage exists, therefore, for an optical scanning method and apparatus capable of OMR processing of sheets that have response targets and key marks printed in the same color. This would allow the response sheets to be printed on less expensive printers and with less exacting print control.